Field of the Invention
This invention relates, in general, to sewing machines and, in particular, to a new and useful sewing machine with an apparatus for adjusting the position of a surface of a needle plate insert relative to a needle plate.
A similar sewing machine is known, for instance, for U.S. Pat. No. 3,513,795. A needle plate insert is fastened to a pivoted first shaft, which has a beveled right-angle bend at one end. The right-angled bend interacts with a first angled end of a second pivoted shaft which has a second end which is linked by a lever to the piston rod of a cylinder. A journal of the piston rod can move in a sliding guideway of the lever.
The linear motion of the piston rod is transmitted to the first shaft by the drive unit. Accordingly, the shaft, together with the needle plate insert, can be switched from one position (sewing) to another position (resting) by a 90 -degree turning motion. In its sewing position, the needle plate insert protrudes beyond the needle plate, while in its resting position it does not.
In sewing machines as discussed above, the linear motion of the piston rod is transmitted to the needle plate insert across several sliding linkages. Rather intense wear occurs at the contact surfaces of these sliding links, due to friction. This produces an additional slackness in the gear mechanism, therefore the vertical height of the needle plate insert can only be approximately adjusted or inexactly adjusted. Further, the needle plate insert executes a swerving movement when in the sewing position under the effect of stress generated by stitch formation. The magnitude of deflections caused by the movement depends on the magnitude of the stress and on the tolerances in the gear mechanism.